docs: def _insert(self, bs, pos):
          """Insert bs at pos."""
          assert 0 <= pos <= self.len
          if pos > self.len // 2:
              # Inserting nearer end, so cut off end.
              end = self._slice(pos, self.len)
              self._truncateend(self.len - pos)
              self._append(bs)
              self._append(end)
          else:
              # Inserting nearer start, so cut off start.
              start = self._slice(0, pos)
              self._truncatestart(pos)
              self._prepend(bs)
              self._prepend(start)
          try:
              self._pos = pos + bs.len
          except AttributeError:
              pass
          assert self._assertsanity()